1. Field of the Invention
The present invention relates generally to wireless communication systems and, in particular, to power measurement in wireless communication systems.
2. Description of the Related Art
Wireless communication systems employ Code Division Multiple Access (“CDMA”) modulation techniques to permit a large number of system users to communicate with one another. The ability of such systems to work is based on the fact that each signal is coded with spreading sequences, such as pseudo-random noise (“PN”) sequences, and orthogonal spreading sequences such as Walsh codes. This coding permits signal separation and signal reconstruction at the receiver. In typical CDMA systems, communication is achieved by using a different spreading sequence for each channel. This results in a plurality of transmitted signals sharing the same bandwidth. Particular transmitted signals are retrieved from the communication channel by despreading a signal from all of the signals by using a known user despreading sequence related to the spreading sequence implemented at the transmitter.
FIG. 1 illustrates CDMA system 100. The geographic area serviced by CDMA system 100 is divided into a plurality of spatially distinct areas called “cells.” Although cells 102, 104, 106 are illustrated as a hexagon in a honeycomb pattern, each cell is actually of an irregular shape that depends on the topography of the terrain surrounding the cell. Each cell 102, 104, 106 contains one base station 112, 114, and 116, respectively. Each base station 112, 114, and 116 includes equipment to communicate with Mobile Switching Center (“MSC”) 118, which is connected to local and/or long-distance transmission network 120, such as a public switch telephone network (PSTN). Each base station 112, 114, and 116 also includes radios and antennas that the base station uses to communicate with mobile terminals 122, 124.
When a call is set up in a CDMA system, a base station and mobile terminal communicate over a forward link and a reverse link. The forward link includes communication channels for transmitting signals from the base station to the mobile terminal, and the reverse link includes communication channels for transmitting signals from the mobile terminal to the base station. The base station transmits certain types of control information to the mobile terminal over a communication channel, referred to herein as a forward-link control channel, also known in the art as a forward overhead channel. Forward-link control channels include the pilot, paging, and synchronization channels. The base station transmits voice or data, and certain types of control information over a communication channel, referred to herein as a forward-link traffic channel. The signals on the communication channels are organized in time periods, referred to herein as frames. Frames are typically 20-millisecond (ms) in length. The signals transmitted over the control channels are referred to herein as control signals, and the signals transmitted over the traffic channels are referred to herein as traffic signals.
FIG. 2 shows a portion of base station 112. Base station 112 includes master controller 130, channel units 140 and 142, radio unit 150 that includes a baseband transmit and receive section, and an RF section. Base station 112 also includes amplifier 160, peripheral hardware 170, and antenna 180. Although only two channel units are shown, the base station can include either more or less channel units.
Each channel unit 140 and 142 includes channel unit controllers 196 and 198, respectively, and multiple channel elements 184, 186, 188 and 190, 192, 194, respectively. A channel element is required for each call being handled by the base station. The channel elements encode the data in a signal with the spreading codes. Each signal transmitted by base station 112 is the output of one of the channel elements. The outputs of the channel elements are digitally combined to form a combined-baseband signal. The combined-baseband signal is then provided as an input into the radio unit 150 where the signal is slightly amplified and is modulated onto a carrier signal. The modulated signal is amplified by amplifier 160, and then transmitted via antenna 180 to mobile terminal 122.
One of the resource management problems that base station 112 must consider is how to manage forward link power. Base station 112 must determine how much power is being transmitted, such that the maximum power output of the base station amplifiers is not exceeded when additional power is requested.
Base station 112 may implement two forms of power control: individual power control for each traffic signal, which is implemented by most base stations, and overload power control, which is implemented by only some base stations.
In the case of the individual power control, base station 112 determines the power level of the forward link signals to each mobile terminal independent of the power levels of the forward-link signals of the other mobile terminals communicating with the base station. Each mobile terminal receives the transmitted signal and obtains the forward-link signal intended for that mobile terminal. When mobile terminal 122 receives a traffic signal, in CDMA systems that comply to the IS-95 standard, mobile terminal 122 checks to determine whether the received forward-link traffic frame is in error. In a subsequent reverse-link traffic frame that mobile terminal 122 transmits, mobile terminal 122 indicates to base station 112 whether there was an error. In CDMA 2000 systems, mobile terminal 122 checks to determine whether the received forward-link traffic signal has sufficient signal strength to overcome the noise in the system, typically by checking the forward-link traffic signal's signal-to-noise ratio. Mobile terminal 122 then indicates to base station 112 whether the forward-link traffic signal strength is sufficient.
Upon receiving from mobile terminal 122 the information of whether there was an error (in IS-95 compliant CDMA systems) or whether the forward-link traffic signal strength is sufficient (in CDMA 2000 systems) base station 112 determines whether its forward link to this mobile terminal is in fading. Base station 112 then adjusts the power level of this signal accordingly. For example, if base station 112 receives one or more successive indications that there is an erred forward-link traffic frame, base station 112 may determine that this forward-link traffic channel is in fading and increase the power level of this signal.
A goal of overload power control is to ensure that the total power transmitted by base station 112 does not exceed the power level at which the base station's equipment is designed to operate over an extended time period. Overload power control is beneficial when the number of signals that can be transmitted simultaneously by the base station is limited by the total power of all the transmitted signals. The total power of all the transmitted signals is a limiting factor for the number of signals that can be transmitted simultaneously because there is maximum output power level at which the amplifier 160 is designed to operate over an extended time period. This maximum output power is referred to herein as the amplifier's maximum continuous power level. When the power level reaches the amplifier's maximum continuous power level an overload condition exists. The base station should determine how much total power is being transmitted, such that the amplifier's maximum continuous power level is not exceeded for a significant time period when additional power is requested.
Conventional CDMA systems that have some form of overload power control may still not be able to solve the problem of reaching an overload condition. The overload power control may fail to prevent base station 112 from reaching an overload condition due to the rapid variations in the power demand of individual traffic channels and the relatively slow feedback used to provide overload power control. Master controller 130 typically obtains the total power level of the signals amplified by amplifier 160 to determine if there is an overload condition.
Master controller 130 obtains the total power level by obtaining from the channel units 140 and 142 the power level of each signal. Channel units 140 and 142 obtains the power level of each signal using the gain and the information rate of the signal. Each channel unit 140 and 142 receives the gain and the information rate of the signal from each channel element under its control. Channel units 140 and 142 receive the gain and information rate when either: 1) there is change in the gain of the signal corresponding to that channel element; or 2) there has not been a change in the gain for an extended time period, such as 1 second. The information rate is the number of bits of information transmitted per second. Four different information rates are possible: full rate, ½ rate, ¼ rate, and ⅛ rate. The information rate is at the full rate when a large amount of information is being transmitted from the base station to the mobile terminal, and it is at the ⅛ rate when a small amount of information is being transmitted from the base station to the mobile. The ½ and the ¼ rate are transitional rates. On the forward link, when the information rate is ½, ¼, or ⅛, the channel rate, which is the set number of voice or data bits that are transmitted within each frame, is higher than the information rate, and the information is repeated several times per frame. For example, with the ½ rate information is repeated twice each frame; with the ¼ rate the information is repeated four times per frame; and with the ⅛ rate the information is repeated eight times per frame. Repeating the information several times per frame permits the information to be transmitted at a correspondingly lower power.
For each channel element, channel unit controllers 140 and 142 average the information rate over an extended predetermined time period and multiply the averaged information rate by the last reported gain squared to obtain the power level of the signal associated with the channel element. Each channel unit controller 140 and 142 then sends the power level to master controller 130, which sums the power levels of all the channel elements communicating with it to obtain the total power level. Master controller 130 then compares the total power level with the amplifier's maximum continuous power level to determine if there is an overload condition. If master controller 130 determines that there is an overload condition, the base station implements one of several remedies. These remedies typically include: a) denying access to any new call requests, referred to herein as call blocking; b) restricting the transmitted signal to its current level; or c) even clipping the transmitted signal.
Problems arise because, as described above, the channel unit controllers 140 and 142 average the information rate over the extended predetermined time period, typically several seconds, and multiply the averaged information rate by the last reported gain squared. The gain may have changed several times during the extended predetermined time period. Therefore, the power levels obtained by channel unit controllers 140 and 142 may not be the actual power levels of the signals. Master controller 130 uses these power levels to obtain the total power level of the signals, and therefore, this obtained total power level may not be the power level being transmitted by base station 112. However, it is this total power level that master controller 130 compares with the amplifier's maximum continuous power level to determine if there is an overload condition. Therefore, master controller 130 may not be able to accurately determine when there is an overload condition. As a result, conventional base stations can exceed amplifier 160's maximum continuous power level and thereby damage amplifier 160. These drawbacks of conventional base stations have required an over-design of the amplifiers to cope with the overload conditions. Extra margin on the amplifier translates into more expensive and larger systems that still do not guarantee that the base station will perform without overloads.